Downhole tool utilizing opposed pistons

ABSTRACT

A subsurface safety valve has a tubular valve housing, a valve closure member movable between an open and a closed position, an axially movable opening flow tube for opening the valve closure member. Hydraulic pressure from a control line is used to move a first piston, which in turn moves the axially movable opening prong through the closure member. A balance line can be used to provide a hydrostatic pressure against a second piston. The first and second pistons are coupled to a sliding member within the valve housing. Thus, the opposed piston arrangement substantially offsets any downward force on the first piston.

BACKGROUND OF THE INVENTION

[0001] 1. Technical Field of the Invention

[0002] The present invention relates to a downhole tool such as asubsurface safety valve and, more particularly, to a subsurface safetyvalve having a tubular housing and an axially shiftable flow tube usedto manipulate a valve closure member.

[0003] 2. Description of Related Art

[0004] Subsurface safety valves (SSSVs) are used within well bores toprevent the uncontrolled escape of well bore fluids, which if notcontrolled could directly lead to a catastrophic well blowout. Certainstyles of safety valves are called flapper type valves because the valveclosure member is in the form of a circular disc or in the form of acurved disc. These flappers can be opened by the application ofhydraulic pressure to a piston and cylinder assembly to move an openingprong against the flapper. The opening prong is biased by a helicalspring in a direction to allow the flapper to close in the event thathydraulic fluid pressure is reduced or lost.

[0005]FIGS. 1a and 1 b illustrate a standard safety valve configurationwherein a safety valve 10 is interposed in a tubing string 12. A controlline 16 is used to open the valve. The valve 10 includes a tubular valvehousing 14 with an axial passage 20. When hydraulic pressure is appliedthrough port 22, the pressure forces a piston 24 to engage an axiallyshiftable control rod 26 which is coupled to an opening prong 30. As thepressure forces the piston downward, the opening prong 30 engages theclosure member 32 and pushes the member into an open position. A spring28 opposes the motion of the piston so that when the hydraulic pressureis released, the piston and opening prong 30 are returned to a firstposition. The weight of the hydraulic fluid produces a hydrostatic“head” force against the piston, and thus is a factor in sizing thespring 28. In general, the pressure required to close the valve 10 isgiven by:

Pressure_(closing)=Force_(spring)/Area_(piston)

[0006] Setting subsurface safety valves deeper is typically just amatter of ensuring sufficient closing pressure to offset the hydrostaticpressure acting to cause the valve to stay open. Increasing closingpressure is accomplished by increasing the Force_(spring) term ordecreasing the Area_(piston) term.

[0007] As the valve closing pressure increases, so does the valveopening pressure. The surface capacity to provide operating pressure isa combination of the pressure needed to open the valve and the wellborepressure:

Pressure_(surface)=Pressure_(opening)+Pressure_(well)

[0008] However, the umbilical line used to deliver the hydraulicpressure can limit the available surface operating pressure. Thus, ifthe surface pressure is fixed and the well pressure increases withdepth, the opening pressure decreases with depth. To compensate forchanges in pressure, the valve requires changes in the spring force orpiston area in accordance with the above formulas, thus requiringcustomization of the valve depending on the depth at which it will beplaced. Design considerations of the well, string, and tools involvedcan also make such valve designs impractical at lower well depths.

[0009] For these reasons, designs which operate independent of wellpressure are required. Two well-known designs are the dome chargessafety valves and balance lines safety valves. A balance line valve 40having a piston 48 in a housing 42 is illustrated in FIG. 2. Twohydraulic chambers are pressurized on opposite sides of the piston 48. Acontrol line is coupled to a first port 44 while the balance line iscoupled to a second port 46. Each hydraulic line is filled with the sametype of fluid. Hydrostatic pressure above and below the piston is equal.Thus, there is no downward force on the spring as a result of thehydrostatic pressure. The valve is operated by pressurizing the upperchamber. This increases the downward force, displacing fluid from thelower chamber and compressing the spring 50 to open the valve. Wellpressure only has access to the seal diameters with cross sectionalareas A and A′.

[0010] Well pressure acts upwards on A′ and downwards on A. A and A′ areequal, therefore well pressure has no upward or downward force on thepiston as long as the seals at A and A′ remain intact. Control linepressure acts downward on B-A while balance line pressure acts upward onB-A′. Thus, the hydrostatic pressures on opposite sides of the piston 48are equalized. If seal 52 fails, well pressure enters the balancepressure chamber, acting on B-A, and increasing F3. If the well pressureis great, it may be impossible to supply sufficient surface pressure tothe control line to force the opening prong downward. Thus, the safetyvalve fails to a closed position. If seal 54 fails, well pressure wouldenter the control chamber and act on B-A′, increasing F1. Withoutapplying control line pressure, F1 could be greater than F2+F3. If F1 isgreater than F2+F3, this imbalance causes the valve to fail in an openposition. The valve can be closed by pressuring up the balance line sothat F3+F2 is greater than the well assisted F1. This is only possibleif sufficient balance line pressure can be applied. Another failure modeoccurs when gas in the well fluid migrates into the balance line,reducing the hydrostatic pressure applied by the balance line, i.e.reducing F3.

[0011] Another style of balance line safety valve is illustrated in FIG.3. The valve 60 has a piston 64 captured within a housing 62 and threehydraulic chambers 68, 70, and 72, two above and one below the valvepiston 64. Two control lines are run to the surface. Well pressure actson seals 74, 80. Since the piston areas A and A′ are the same, wellpressure has no influence on the pressure required to displace thepiston. Control line and balance line hydrostatic pressures act onidentical piston areas B-A′ and B-A″, so there is no net upward ordownward force. If seal 74 leaks, well pressure accesses the balanceline system. This pressure acts on area B-A″, boosting force F3, whichwith F2 will overcome F1, to close the valve. If seal 76 leaks,communication between the control and balance lines will be established.F1 will always equal F3. Thus, F2 will be the only active force causingthe valve to close. If seal 78 leaks, it has the same effect as seal 76leaking. If seal 80 leaks, tubing pressure accesses the balance linesystem. This pressure acts to increase F3, overcoming F1 and closing thevalve. Thus, if sufficient control line pressure is available and tubingpressure is relatively low, it may be possible to open the valve ifseals 72 and/or 80 leak. Control line force F1 is greater than thetubing assisted balance force F3 with the spring force F2. In all modesof failure for this valve, the valve fails permanently to a closedposition.

[0012] A dome charge safety valve uses a captured gas charge. The gascharge provides a heavy spring force to achieve an increased closingpressure. However, dome charge designs are complex and requirespecialized manufacturing and personnel. This increases the cost anddecreases the reliability of the design because numerous seals arerequired. Also, industry standards favor metal-to-metal (MTM) sealingsystems. Gas charges require the use of elastomeric seals.

[0013] A need exists for a safety valve suitable for deep setting depthapplications and which is well pressure insensitive. Thus, it shouldincorporate the benefits of a balance line SSSV. Such a design shouldutilize a metal-to-metal sealing system for increased reliability andalso allow for the application of balance line pressure to cycle thevalve's flow tube, thus opening the valve. Further, the design shouldminimize operational friction to reduce the required spring force toclose the valve.

SUMMARY OF THE INVENTION

[0014] The present invention relates to an improved method of actuatinga downhole safety valve that uses a pair of opposed pistons connected toindividual control lines that are run to the surface. The hydrostaticpressure in the control line and balance line affects both pistonsequally, thereby canceling out any net affect. The pistons are situatedin the valve housing so that one will tend to ascend in reaction to thehydrostatic pressure, while the other piston will tend to descend. Bothare coupled to a common axially movable member within the valve.

[0015] To open the valve, the control line attached to the first pistonis pressurized. The increased pressure forces the piston downward untilit rests against a downstop. The distal end of the piston is attached toan axially movable flow tube that pushes through the closure member ofthe valve thereby opening the valve's central passage. A compressionspring opposes the motion of the piston. Therefore, when the openingpressure subsides, the compression spring will return the flow tube toits original position, allowing the closure member to close.

[0016] This arrangement allows for the isolation of the valve fromeffects of hydrostatic pressure and wellbore pressure. It also providesa method of positively closing the valve in the event of a failure. Thepresent invention also uses MTM and non-elastomeric sealing.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017] The novel features believed characteristic of the invention areset forth in the appended claims. The invention itself however, as wellas a preferred mode of use, further objects and advantages thereof, willbest be understood by reference to the following detailed description ofan illustrative embodiment when read in conjunction with theaccompanying drawings, wherein:

[0018]FIGS. 1a and 1 b illustrate a prior art safety valve having asingle control line; FIG. 2 illustrates a balance line safety valvehaving a balance line; FIG. 3 illustrates an improved prior art balanceline safety valve; FIGS. 4a, 4 b and 4 c are sectional views of anembodiment of the present invention with the closure member in theclosed position; and

[0019]FIGS. 5a, 5 b, and 5 c are sectional views of the presentinvention with the closure member in an open position.

DETAILED DESCRIPTION OF THE DRAWINGS

[0020]FIGS. 4a, 4 b and 4 c provide partial sectional views along thelength of a safety valve 100 that embodies the present invention. Thesafety valve 100 has an outer tubular housing that defines a centralpassage 122. The outer housing can be constructed of several sections102, 104, 106, and 108. Each section can be coupled by threadedconnection during the construction of the valve 100. The housing definesa number of inner structures, including piston chambers 110 and 112. Thepiston chamber 110 is coupled to control line 114, while piston chamber112 is coupled to balance line 116. Both the control line and thebalance line can be coupled to a surface pressure source. The weight ofthe hydraulic fluid in the control line and the balance line produces ahydrostatic force within the chambers 110, 112.

[0021] Pistons 118 and 120 are captured in chambers 110, 112respectively. In a static situation, the hydraulic fluid in the controlline and the balance line should exert a substantially equal andoffsetting force on the pistons. To open the valve, only control line114 is pressurized. When sufficient pressure is applied, piston 118moves downward compressing spring 128. The piston simultaneously acts onmovable member 124 and opening prong 130. The pistons are both coupledto a movable member 124. Thus, when piston 118 descends in chamber 110,piston 120 also descends in chamber 112. Likewise, the distal end 130 aof the opening prong 130 contacts the closure member 132 of the valve.The closure member 132 is hinged at 134, allowing it to pivot to an openposition. The piston 118 can travel between upstop 140 and downstop 142.

[0022]FIGS. 5a, 5 b, and 5 c illustrate the closure member in the openposition. Note that the spring 128 is shown in a compressed state. Theclosure member 132 is in an open position, allowing well fluids to passthrough the central passage of the valve. The distal end 130 a of theopening prong rests against a stop 136. In the event that the safetyvalve becomes stuck in an open state, pressure can be applied to thebalance line 116, thus moving piston 120 upward in chamber 112. Themovable member 124 and piston 118 also move upward in conjunction withthe piston 120. The ability to cycle the motion of the valve through theuse of the balance line is an improvement over prior art valves.

[0023] Although preferred embodiments of the present invention have beendescribed in the foregoing Detailed Description and illustrated in theaccompanying drawings, it will be understood that the invention is notlimited to the embodiments disclosed, but is capable of numerousrearrangements, modifications, closure member types and substitutions ofsteps without departing from the spirit of the invention. Accordingly,the present invention is intended to encompass such rearrangements,modifications, closure member types and substitutions of steps as fallwithin the scope of the appended claims. This embodiment is not limitedto tubing conveyed safety valves, but encompasses wireline-conveyedsafety valves, sliding side door devices, and other downhole tools thatare movable.

We claim:
 1. A downhole tool having an element movable by a piston forcebetween a first and second position comprising: (a) a valve housing; (b)a first piston movable within the housing; (c) a second piston movablewithin the housing, wherein the first and second pistons are coupled toeach other.
 2. The downhole tool of claim 1 further comprising: (d) avalve closure member captured in the housing and movable between an openand closed position; (e) an axially shiftable flow tube captured in thehousing for opening the valve closure member.
 3. The downhole of claim 1wherein the first piston is coupled to a control line.
 4. The downholetool of claim 3 wherein the second piston is coupled to a balance line.5. The downhole tool of claim 2 further comprising: (f) a spring withinthe housing and opposing the motion of the axially shiftable flow tube.6. The downhole tool of claim 1 wherein the first and second pistons arecoupled to a movable member.
 7. The downhole tool of claim 1 furthercomprises first and second control lines coupled to a surface pressuresource.
 8. The downhole tool of claim 4 wherein the valve housingcomprises a first and second piston chamber for capturing the first andsecond pistons, and wherein the control line and balance line arecoupled to the valve housing to move the pistons in opposite directions.9. The downhole tool of claim 4 wherein the valve housing comprises afirst and second piston chamber for capturing the first and secondpistons, and wherein a hydrostatic pressure is applied by the balanceline to the second piston which is substantially equal and offsetting toa hydrostatic pressure applied by the control line to the first piston.10. A method of operating a downhole tool placed in the flow path of awell tubing string within a well, comprising the steps of: (a) couplinga control line to a first piston chamber having a first piston; (b)coupling a balance line to a second piston chamber having a secondpiston; and (c) coupling both the first and second pistons to a movablemember.
 11. The method of claim 10 further comprising: (d) supplying asubstantially equal hydrostatic pressure through both the control lineand the balance line.
 12. The method of claim 10 further comprising: (d)supplying a sufficient pressure through the control line to move thefirst piston downward.
 13. The method of claim 12 further comprising:(e) overcoming an opposing spring force; and (f) forcing an openingprong through a closure member.
 14. A safety valve for use in a wellcomprising: (a) a valve housing; (b) a first piston movable within thehousing; (c) a second piston movable within the housing; wherein thefirst and second pistons are coupled to each other; (d) a valve closuremember captured in the housing and movable between an open and closedposition; (e) an axially shiftable flow tube captured in the housing foropening the valve closure member.
 15. The safety valve of claim 14wherein the first piston is coupled to a control line.
 16. The safetyvalve of claim 14 wherein the second piston is coupled to a balanceline.
 17. The safety valve of claim 14 wherein the valve housingcomprises a first and second piston chamber for capturing the first andsecond pistons, and wherein the control line and balance line arecoupled to the valve housing to move the pistons in opposite directions.18. The safety valve of claim 14 wherein the valve housing comprises afirst and second piston chamber for capturing the first and secondpistons, and wherein a hydrostatic pressure is applied by the balanceline to the second piston which is substantially equal and offsetting toa hydrostatic pressure applied by the control line to the first piston.